Traditionally the compressed air inlet pressure of a compressed air spray gun is adjusted to suit a particular compressed air spray gun air cap or spraying medium by a compressed air pressure reducing valve and air pressure gauge mounted upstream of the compressed air inlet, mounted either adjacent to the source of the compressed air near a spray booth or mounted pendant of the handle of the compressed air spray gun. One problem associated with the aforementioned arrangements of air pressure gauge and compressed air spray gun is that the location of the air pressure gauge is such that the air pressure upstream of the compressed air pressure reducing valve is measured. It is preferable to measure the air pressure downstream of the compressed air pressure reducing valve and all other compressed air valves which can affect a variable pressure reduction and lead to inconsistent compressed air pressure reaching the air cap.
It is possible for a compressed air inlet pressure reducing valve to be included as part of the spray gun thus eliminating the need for a separate compressed air pressure reducing valve, however a problem with this arrangement is that there is at present no means of displaying the compressed air pressure, and if a separate compressed air pressure reducing valve is used upstream with an air pressure gauge, the pressure reducing valve is downstream and if not fully open can cause the pressure setting to vary.
A solution to the aforementioned problem is provided by EP 526 525 which discloses a compressed air spray gun comprising a miniature digital electronic air pressure gauge located down stream of the compressed air inlet pressure reducing valve for measuring the air pressure at the air cap and downstream of the trigger operated shut off valve. As such compressed air spray guns are often used in hazardous areas, the air pressure gauge comprises a sealed unit with an aperture leading to a pressure sensitive transducer connected to an amplifier. The signal from the amplifier is input to an analogue digital converter and thence to a decoder/driver and display. On exhaustion of the cell powering the amplifier, the sealed unit is discarded and replaced with a new sealed unit.
The miniature digital electronic air pressure gauge of EP 526 525 is threaded into a threaded aperture bored into the compressed air spray gun. The air pressure gauge is arranged to be in communication with a bore leading to a region downstream of the compressed air inlet pressure reducing valve and downstream of the trigger operated shut off valve.
A similar solution is offered by the ITW OMX-610 spray gun except the threaded aperture receives an analogue pressure gauge and communicates with a region downstream of the compressed air inlet pressure reducing valve but upstream of the trigger operated shut off valve.
DE 100 31 857 A and DE 100 31 858 A disclose spray guns comprising pressure gauges which are integrated into respectively the handle of the spray gun and the compressed air inlet valve. Embodiments are disclosed with the compressed air pressure being measured upstream of the compressed air inlet, between the compressed air inlet pressure reducing valve and the trigger operated shut off valve and downstream of both the compressed air inlet pressure reducing valve and the trigger operated shut off valve.
Given the foregoing, a need has been identified to provide an air pressure gauge which can be retrofitted to a wide range of existing compressed air spray guns without the need to modify the compressed air spray gun body and thereby measure the air pressure downstream of a compressed air inlet pressure reducing valve and trigger operated shut off valve where the mounting of the gauge does not affect the ease of use of the existing spray gun controls.